In signal transmission in a high frequency region, it is usually desired to improve the transmission rate and to reduce noises, and studies are being made with respect to the board materials for printed wiring boards, the wiring techniques, the circuit configurations, etc.
In a printed wiring board, the signal transmission rate is inversely proportional to the square root of the dielectric constant of the board material for the printed wiring board, and it is possible to increase the transmission rate and reduce noises if a low dielectric constant material is employed as the board material. Further, it is possible to reduce an unnecessary capacitance to be generated between adjacent circuits by using a board material having a low dielectric constant. Accordingly, a board material having a low dielectric constant is suitable for a printed wiring board handling a very weak high speed signal in e.g. a high speed digital circuit or an amplifier circuit in a microwave transceiver circuit (JP-A-2-42786).
A printed wiring board made of an epoxy resin is the most common printed wiring board, but the dielectric constant is high and cannot be applied to a high frequency region. Such a printed wiring board can easily be produced by a step of impregnating a glass fiber woven fabric with an epoxy resin, followed by drying, a step of bringing the impregnated epoxy resin to a semi-cured state to form a prepreg, and a step of laminating an electrically conductive metal foil on the prepreg.
A fluororesin such as polytetrafluoroethylene (hereinafter referred to as PTFE) has characteristics such that the dielectric constant and dielectric loss tangent are low. However, the adhesion between the fluororesin and the metal is not sufficient, and it has been difficult to produce a printed wiring board made of a fluororesin by such a simple process for the above-mentioned printed wiring board made of an epoxy resin. Usually, a printed wiring board of a fluororesin is produced by bonding an electrically conductive metal foil and an electrical insulator layer made of a PTFE-impregnated glass fiber fabric obtained by impregnating a glass fiber fabric with an aqueous dispersion of e.g. PTFE, followed by baking, via an adhesive layer made of e.g. a low melting point thermosetting resin, an adhesive film or an adhesive agent (JP-A-11-199738).
However, the printed wiring board of a fluororesin produced by the above process, has an adhesive layer having high dielectric constant and dielectric loss tangent, whereby the characteristics of the fluororesin i.e. the low dielectric constant and dielectric loss tangent will be lost. If such a printed wiring board is applied to a high frequency region, the transmission loss tends to be very high.
Further, in a high frequency region, a skin effect will appear. The skin effect is a phenomenon such that a high frequency electrical current will flow only in the vicinity of the surface of the electrical conductor layer. For example, the electric current will flow only within a range of 2.3 μm in depth from the surface with 1 GHz or within a range of 0.7 μm in depth from the surface with 10 GHz.
Also in a case where an adhesive is employed, in order to improve the adhesion between the electrical insulator layer and the electrical conductor layer, irregularities of about 3 μm are provided on the surface of the electrical conductor layer on the side being in contact with the electrical insulator layer, but there will be a deviation in the signal arrival time between the surface having such irregularities (hereinafter referred to also as a roughened surface) and a non-roughened surface. Therefore, such irregularities are required to be profiled as low as possible (JP-A-5-55746).
Under the circumstances, it is desired to develop a printed wiring board of a fluororesin which requires no adhesive layer and in which the irregularities on the electrical conductor layer surface are low-profiled, and the electrical conductor layer and the electrical insulator layer are directly bonded.